1. Field of the Invention
The present general inventive concept relates to an image forming apparatus such as a printer, a facsimile, or a copier, and a method thereof, and more particularly, to an electro-photographic image forming apparatus and a method thereof.
2. Description of the Related Art
An electro-photographic image forming apparatus scans a laser beam to a surface of a photosensitive medium charged with a predetermined electrical potential and forms an electrostatic latent image thereon. After the electrostatic latent image is developed with a predetermined color toner, a complete color image is formed on a surface of a transfer belt as respective color toner images are transferred to the transfer belt to be sequentially formed on one another. A sheet of printing paper stacked in a paper cassette is delivered to a transfer roller rotating in contact with the transfer belt by a pick-up roller, and the color image is transferred thereto. The color image transferred onto the printing paper is fixed on the printing paper by a fixing part. The printing paper on which the image is fixed is discharged outside the image forming apparatus.
FIG. 1 schematically shows a conventional electro-photographic image forming apparatus.
Referring to FIG. 1, the electro-photographic image forming apparatus includes an image forming unit having a plurality of photosensitive media 10 to be charged, a light exposing unit 20 scanning a laser beam onto a surface of each of the plurality of photosensitive media 10 and forming a predetermined electrostatic latent image thereon, and a plurality of developing units 30 developing the electrostatic latent image with color toners, a transfer belt 40 being supported by a drive roller 41 and a steering roller 42 to rotate endlessly in a closed loop while being in contact with the plurality of photosensitive media 10, a transfer voltage applying unit 50 applying a predetermined transfer voltage to the transfer belt 40 for transferring a toner image formed on the surface of each of the plurality of photosensitive media 10 to the transfer belt 40, and a transfer roller 60 being disposed outside the transfer belt to correspond to the drive roller 41 for transferring the toner image transferred onto the surface of the transfer belt 40 to a sheet of printing paper P delivered from a paper cassette (not shown).
The transfer voltage applying unit 50 has a plurality of voltage applying rollers 51 and a voltage applying part 52. The plurality of voltage applying rollers 51 are arranged inside the transfer belt 40 to be in contact with an inner surface thereof and to correspond to each of the plurality of photosensitive media 10. A transfer voltage having an opposite polarity of the toner is applied from the voltage applying part 52 to the plurality of the voltage applying rollers 51.
Therefore, the toner image formed on each of the plurality of photosensitive media 10 is transferred to the transfer belt 40 by an electrostatic force generated between each of the plurality of photosensitive media 10 and the transfer belt 40 by the transfer voltage applying unit 50. When toner images transferred as mentioned above are sequentially formed on one another, a complete color image is formed on the surface of the transfer belt 40. The color image formed on the transfer belt 40 is transferred to the printing paper P by a mechanical force generated between the drive roller 41 and the transfer roller 60 and an electrostatic force generated between the transfer belt 40 and the transfer roller 60.
Here, the transfer voltage applied when the toner images are transferred from the plurality of photosensitive media 10 to the transfer belt 40, is approximately −1 KV, when a plus (positive) toner having a positive potential is used. At this time, an electrical potential of the transfer belt 40 is 0 KV. The toner image on the surface of each of the plurality of photosensitive media 10 is transferred to the transfer belt 40 by the electrical potential difference between the transfer belt and the toner.
However, in the conventional electro-photographic image forming apparatus as described above, the electrical charge (a positive electrical charge when the positive toner is used, a negative electrical charge when negative toner is used) of the photosensitive medium 10 moves to the transfer belt 40 and accumulates on the surface of the transfer belt 40, when the transfer voltage is applied to transfer the toner images from the plurality of photosensitive media 10 to the transfer belt 40. During printing, the electrical charge is accumulated on the surface of the transfer belt 40. As shown in FIG. 3A, the surface electrical potential of the transfer belt 40 is changed as time passes. As a result, the transfer voltage is decreased as the printing operation continues. Therefore, there is a problem of a lowered transferring efficiency and subsequent decrease of an image density.